Answer: Unfortunately, based on your description I am not able to determine the exact cause of the yellowing. It could be caused by a magnesium deficiency which could be remedied by applying Epsom salts at the rate of one Tablespoon to the gallon as a foliar feed and soil drench once a week for several weeks. Container grown plants are subject to deficiencies due to nutrients leaching out of the soil during frequent watering.
It is also possible that the plants have been overwatered. Make sure the soil is kept evenly moist but not sopping wet. If it has been raining you will need to water less accordingly.
It could also be the start of a disease. You might want to take a sample to your county extension (745-3445) for a more specific diagnosis and suggested remedies, if any.
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